In U.S. Pat. No. 4,626,767 to Clappier and assigned to the same assignee as the present invention there is disclosed an alternating current, constant voltage power supply feeding a load through a quarter wavelength circuit to convert the constant voltage to a constant current. The frequency of operation of the supply is radio frequency and the frequency disclosed in the aforesaid patent is 13.56 MHz.
The aforesaid power supply is employed by the assignee Metcal, Inc. to power a line of temperature self regulating heaters such as soldering irons, heater straps and related heaters which rely on the change of magnetic permeability of certain materials at their effective Curie temperatures to control temperature of the heaters. Such materials may be ferromagnetic materials; iron-nickel alloys for instance; ferrites and other high magnetic permeability materials that lose their high magnetic permeability at specific temperatures. The term "effective Curie temperature" refers to the fact that the temperature at which a material becomes non-magnetic for purposes of temperature control may be as little as 1.degree. C. or as much as 100.degree. C. less than absolute Curie temperature.
The underlying principle of these temperature self regulating heaters lies in the fact that due to skin effect produced by alternating currents flowing through or inductively induced in a high permeability conductor particularly at high frequencies, the current is confined to a narrow region of the high permeability (high mu) conductor thus causing it to exhibit a high resistance to current flow. When the material is heated by such current flow to its effective Curie temperature, the skin effect is materially reduced, the current spreads more uniformly throughout the body of the conductor and the resistance to flow is materially reduced. See for instance U.S. Pat. No. 4,256,945.
The power generated in a system as discussed above is defined by P=I.sup.2 R. If I is held constant then P=KR. If now the resistance is materially reduced in the system as effective Curie temperature is approached, the system cannot generate the heat necessary to maintain effective Curie temperature, the conductor cools, the conductor again becomes highly magnetically permeable and the cycle repeats.
As indicated above the current must remain constant to achieve the best possible temperature regulation although some variation in current is permissible if some leeway is allowed in temperature control within a given system.
The power supply disclosed in U.S. Pat. No. 4,626,767 provides quite rigid current control and is expensive, presently in the $350 to $400 range. Attempts to lessen the cost by other approaches such as disclosed in Carter U.S. Pat. No. 4,795,886 and Hall U.S. Pat. No. 4,769,519 have not as yet been developed to the point of commercial use.